1. Field of the Invention
The present invention relates to a valve timing adjusting device which controls timing of opening and closing at least one of an intake valve and an exhaust valve in an internal combustion engine. Hereinafter, the above timing of opening and closing is referred to as valve timing.
2. Description of Related Art
Conventionally, a valve timing adjusting device that controls timing of opening and closing at least one of an intake valve and an exhaust valve in an internal combustion engine has improved the startability of the engine by a phase control for advancing and retarding valve timing at the start of the engine. As a known technique to assure high startability after a failure such as an engine stall, a return spring is mounted in a valve timing adjusting device for the phase control as described in JP-A-2007-138730. Typically, the phase control is performed for adjusting a phase relation between a crankshaft and a camshaft in the internal combustion engine.
However, in a case, where a return spring is mounted over a housing and a vane rotor of the valve timing adjusting device as described in JP-A-2007-138730, a biasing force of the return spring changes between (a) when the vane rotor of the device is positioned at in the most advance position and (b) when the vane rotor is positioned at the most retard position. As a result, the difference of the biasing force influences the phase control caused by working fluid pressure, and thereby making precise phase control difficult.
Because an average cam torque is large at the start of the engine at a very low temperature, the return spring biasing force is required to be increased. However, in a case, where the return spring biasing force is increased, the precise phase control by using the working fluid pressure may become difficult in normal operation of the engine disadvantageously. As a result, in the valve timing adjusting device, the return spring biasing force is required to be set to a level, which does not substantially influence the phase control in normal operation of the engine.